This invention relates to the fastening of articles together into an assembly, and, in particular, to such an assembly wherein the fastener is operable via a solid state phase transformation.
Nearly all structures require the joining and assembly of multiple individual articles. The articles can be joined permanently (e.g., welding, soldering, adhesive bonding), semipermanently (e.g., removable fasteners), or easily separably (e.g., clamps, mechanical engagements). The present invention relates to an assembly made with a permanent or semipermanent fastener.
Various types of removable fastening techniques are available, such as screws, bolts, some types of rivets, and the like. While widely used and operable in many circumstances, available semipermanent fasteners have drawbacks in some specific applications. When Joining articles made of particular materials such as magnesium, nonmetallic composites, and honeycomb structures, care must be taken that the fastener does not damage the article. For example, if the fastener scratches a coated magnesium article, the article may quickly begin to corrode in service at the scratched location. This corroded location may not be readily visible or accessible, as where it is within a bore through which the fastener fits. The result is a structure subject to an unexpected, undetected, accelerated failure.
An alternative approach useful in some situations, typically where a first article has a bore therein and a second article fits tightly within the bore, is shrink fitting. The fastening effect is based upon a frictional engagement between the articles, so that there is little likelihood of scratching and also little opportunity for corrosive agents to penetrate into the joint. In this approach, the first article is heated or the second article is cooled, or both, so that there is a clearance between the second article and the bore due to thermal expansion. The second article is inserted into the bore, and the two articles are allowed to thermally equilibrate to lock them together. The temperature difference that is required to form a joint by this approach is relatively large, inasmuch as thermal expansion coefficients are measured in parts per million per degree.
In theory, the two articles may later be separated by reversing this process. The first article is heated from its outside surface until it expands sufficiently to permit the first article to be removed from the bore. This separation technique is unreliable, because the second article is also heated and expands to fit the bore. The articles can sometimes be separated, but often cannot be separated.
There is a need for an improved technique for fastening two or more articles together, which overcomes the shortcomings of existing approaches. The present invention fulfills this need, and further provides related advantages.